U.S. patent number 8,093,491 [Application Number 11/145,538] was granted by the patent office on 2012-01-10 for lead free solar cell contacts.
This patent grant is currently assigned to Ferro Corporation. Invention is credited to Chandrashekhar S. Khadilkar, Steve S. Kim, Tung Pham, Aziz S. Shaikh, Srinivasan Sridharan.
United States Patent |
8,093,491 |
Sridharan , et al. |
January 10, 2012 |
Lead free solar cell contacts
Abstract
Formulations and methods of making solar cells are disclosed. In
general, the invention presents a solar cell contact made from a
mixture wherein the mixture comprises a solids portion and an
organics portion, wherein the solids portion comprises from about
85 to about 99 wt % of a metal component, and from about 1 to about
15 wt % of a lead-free glass component. Both front contacts and
back contacts are disclosed.
Inventors: |
Sridharan; Srinivasan
(Strongsville, OH), Pham; Tung (Vista, CA), Khadilkar;
Chandrashekhar S. (Broadview Heights, OH), Shaikh; Aziz
S. (San Diego, CA), Kim; Steve S. (Goleta, CA) |
Assignee: |
Ferro Corporation (Cleveland,
OH)
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Family
ID: |
37498896 |
Appl.
No.: |
11/145,538 |
Filed: |
June 3, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060289055 A1 |
Dec 28, 2006 |
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Current U.S.
Class: |
136/256; 136/265;
136/261; 136/252 |
Current CPC
Class: |
H01L
31/022425 (20130101); C03C 8/18 (20130101); C03C
3/066 (20130101); C03C 3/064 (20130101); Y02E
10/50 (20130101) |
Current International
Class: |
H01L
31/042 (20060101) |
Field of
Search: |
;136/243-265 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2006201556 |
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Nov 2006 |
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AU |
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58083073 |
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May 1983 |
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JP |
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Other References
Khadilkar et al., "Characterization of Front Contact in a Silicon
Solar Cell," presented Aug. 10-13, 2003. cited by other .
Shaikh et al., "Designing a Front Contact Ink for SiNx Coated
Polycrystalline Si Solar Cells," presented May 11-18, 2003. cited
by other .
Surek, "Progress in U.S. Photovoltaics: Looking Back 30 Years and
Looking Ahead 20," 3rd World Conference, Osaka, Japan, May 11-18,
2003, pp. 1-6. cited by other.
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Primary Examiner: Hendricks; Keith D.
Assistant Examiner: Dinh; Bach
Attorney, Agent or Firm: Rankin, Hill & Clark LLP
Claims
What is claimed is:
1. A solar cell contact comprising, prior to firing: A silicon
semiconductor wafer; A composition deposited on the silicon
semiconductor wafer, the composition comprising: a. A solids
portion and b. An organics portion, c. Wherein the solids portion
comprises i. From about 85 to about 99 wt % of a conductive metal
component silver, and ii. From about 1 to about 15 wt % of a glass
component, wherein the glass component is lead-free and
cadmium-free, the glass component including from 50 to 80 mol %
Bi.sub.2O.sub.3, from 15 to 35 mol % SiO.sub.2, from 1 to 15 mol %
ZnO and from 1 to 15 mol % V.sub.2O.sub.5; wherein the glass
component includes a second-phase crystalline material selected
from the group consisting of alumino-silicates, bismuth borates,
bismuth silicates, bismuth titanates, vanadates, bismuth vanadates,
bismuth vanadium titanates, zinc borates, zinc titanates, zinc
silicates, zirconium silicates, and reactions products thereof and
combinations thereof.
2. The solar cell contact of claim 1 wherein the silicon
semiconductor wafer includes an N-side and a P-side.
3. The solar cell contact of claim 2 wherein the composition is
deposited on the N-side of the silicon semiconductor wafer.
4. The solar cell contact of claim 2 wherein the composition is
deposited on the P-side of the silicon semiconductor wafer.
5. The solar cell contact of claim 1 wherein the glass component
further comprises from about 0.1 to about 20 mol % of pentavalent
oxide selected from the group consisting of P, Ta, Nb, and Sb.
6. The solar cell contact of claim 1 wherein the second phase
crystalline material is bismuth borate.
7. The solar cell contact of claim 1 wherein the second phase
crystalline material is bismuth silicate and the bismuth silicate
is selected from the group consisting of
6Bi.sub.2O.sub.3.SiO.sub.2, Bi.sub.2O.sub.3.SiO.sub.2,
2Bi.sub.2O.sub.3.3SiO.sub.2, 12Bi.sub.2O.sub.3.SiO.sub.2,
2Bi.sub.2O.sub.3.SiO.sub.2, 3Bi.sub.2O.sub.3.5SiO.sub.2 and
Bi.sub.2O.sub.3.4SiO.sub.2.
8. The solar cell contact of claim 1 wherein the second phase
crystalline material is bismuth titanate and the bismuth titanate
is selected from the group consisting of
Bi.sub.2O.sub.3.2TiO.sub.2, 2Bi.sub.2O.sub.3.3TiO.sub.2,
2Bi.sub.2O.sub.3.4TiO.sub.2, and 6Bi.sub.2O.sub.3.TiO.sub.2.
9. The solar cell contact of claim 1 wherein the second phase
crystalline material is a vanadate and the vanadate is selected
from the group consisting of MgO.V.sub.2O.sub.5,
SrO.V.sub.2O.sub.5, CaO.V.sub.2O.sub.5, BaO.V.sub.2O.sub.5,
ZnO.V.sub.2O.sub.5, Na.sub.2O.17V.sub.2O.sub.5,
K.sub.2O.4V.sub.2O.sub.5, and 2Li.sub.2O.5V.sub.2O.sub.5.
10. The solar cell contact of claim 1 wherein the second phase
crystalline material is bismuth vanadate and the bismuth vanadate
is selected from the group consisting of
6Bi.sub.2O.sub.3.V.sub.2O.sub.5, BiVO.sub.4,
2Bi.sub.2O.sub.3.3V.sub.2O.sub.5, and BiV.sub.3O.sub.9.
11. The solar cell contact of claim 1 wherein the second phase
crystalline material is bismuth vanadium titanate and the bismuth
vanadium titanate is
6.5Bi.sub.2O.sub.3.2.5V.sub.2O.sub.5.TiO.sub.2.
12. The solar cell contact of claim 1 wherein the second phase
crystalline material is zinc titanate and the zinc titanate is
2ZnO.3TiO.sub.2.
13. The solar cell contact of claim 1 wherein the second phase
crystalline material is zinc silicate and the zinc silicate is
ZnO.SiO.sub.2.
14. The solar cell contact of claim 1 wherein the second phase
crystalline material is zirconium silicate and the zirconium
silicate is ZrO.sub.2.SiO.sub.2.
15. The solar cell contact of claim 1 wherein the second phase
crystalline material is zinc borate and the zinc borate is
ZnO*B.sub.2O.sub.3.
16. The solar cell contact of claim 1 wherein the composition is
fired for a time period of from 1 second to 5 minutes.
17. The solar cell contact of claim 16 wherein the composition is
fired for a time period of from 1 second to 3 minutes.
18. The solar cell contact of claim 1 wherein the silver component
comprises silver selected from the group consisting of flakes,
powder, or colloidal particles of silver.
19. The solar cell contact of claim 18 wherein the solids portion
further comprises phosphorus, at least a portion of which is
present as a coating on at least a portion of the silver flakes,
powder, or colloidal particles.
20. The solar cell contact of claim 1 wherein the solids portion
comprises from about 3 to about 8 wt % of the glass component.
21. A solar cell contact comprising, prior to firing: A silicon
semiconductor wafer; A composition deposited on the silicon
semiconductor wafer, the composition comprising: d. A solids
portion and e. An organics portion, f. Wherein the solids portion
comprises i. From about 85 to about 99 wt % of a conductive metal
component silver, and ii. From about 1 to about 15 wt % of a glass
component, wherein the glass component comprises: 15 to 80 mol %
Bi.sub.2O.sub.3; 2 to 45 mol % SiO.sub.2; 0.1 to 25 mol % ZnO; and
0.1 to 25 mol % V.sub.2O.sub.5; wherein the glass component
includes a second-phase crystalline material selected from the
group consisting of alumino-silicates, bismuth borates, bismuth
silicates, bismuth titanates, vanadates, bismuth vanadates, bismuth
vanadium titanates, zinc borates, zinc titanates, zinc silicates,
zirconium silicates, and reactions products thereof and
combinations thereof.
22. The solar cell contact of claim 21 wherein the glass component
comprises: 50 to 80 mol % Bi.sub.2O.sub.3; 15 to 35 mol %
SiO.sub.2; 1 to 15 mol % ZnO; and 1 to 15 mol % V.sub.2O.sub.5.
wherein the silver component comprises silver selected from the
group consisting of flakes, powder, or colloidal particles of
silver, wherein the solids portion further comprises phosphorus, at
least a portion of which is present as a coating on at least a
portion of the silver flakes, powder, or colloidal particles.
23. The solar cell contact of claim 21 wherein the glass component
further comprises from about 0.1 to about 20 mol % of pentavalent
oxide selected from the group consisting of P, Ta, Nb, and Sb.
24. The solar cell contact of claim 21 wherein the glass component
consists essentially of: 15 to 80 mol % Bi.sub.2O.sub.3; 2 to 45
mol % SiO.sub.2; 0.1 to 25 mol % ZnO; and 0.1 to 25 mol %
V.sub.2O.sub.5.
25. The solar cell contact of claim 21 wherein the glass component
consists essentially of: 50 to 80 mol % Bi.sub.2O.sub.3; 15 to 35
mol % SiO.sub.2; 1 to 15 mol % ZnO; and 1 to 15 mol %
V.sub.2O.sub.5.
26. A solar cell contact comprising, prior to firing: A silicon
semiconductor wafer; A composition deposited on the silicon
semiconductor wafer, the composition comprising: a. A solids
portion and b. An organics portion, c. Wherein the solids portion
comprises i. From about 85 to about 99 wt % of a conductive metal
component silver, and ii. From about 1 to about 15 wt % of a glass
component, wherein the glass component comprises: 10 to 40 mol %
Bi.sub.2O.sub.3; 30 to 65 mol % SiO.sub.2; 3 to 20 mol %
B.sub.2O.sub.3; and 5 to 25 mol % alkali oxides; wherein the glass
component includes a second-phase crystalline material selected
from the group consisting of alumino-silicates, bismuth borates,
bismuth silicates, bismuth titanates, vanadates, bismuth vanadates,
bismuth vanadium titanates, zinc borates, zinc titanates, zinc
silicates, zirconium silicates, and reactions products thereof and
combinations thereof.
27. The solar cell contact of claim 26 wherein the glass component
consists essentially of: 10 to 40 mol % Bi.sub.2O.sub.3; 30 to 65
mol % SiO.sub.2; 3 to 20 mol % B.sub.2O.sub.3; and 5 to 25 mol %
alkali oxides.
Description
FIELD OF THE INVENTION
This invention relates to lead-free and cadmium-free paste
compositions and a method of making contacts for solar cells as
well as other related components used in fabricating photovoltaic
cells.
BACKGROUND
Solar cells are generally made of semiconductor materials, such as
silicon (Si), which convert sunlight into useful electrical energy.
Solar cells are, in general, made of thin wafers of Si in which the
required PN junction is formed by diffusing phosphorus (P) from a
suitable phosphorus source into a P-type Si wafer. The side of the
silicon wafer on which sunlight is incident is generally coated
with an anti-reflective coating (ARC) to prevent reflective loss of
sunlight, which increases the solar cell efficiency. A two
dimensional electrode grid pattern known as a front contact makes a
connection to the N-side of silicon, and a coating of aluminum (Al)
makes connection to the P-side of the silicon (back contact).
Further, contacts known as silver rear contacts, made out of silver
or silver-aluminum paste are printed and fired on the N-side of
silicon to enable soldering of tabs that electrically connect one
cell to the next in a solar cell module. These contacts are the
electrical outlets from the PN junction to the outside load.
Conventional pastes for solar cell contacts contain lead frits.
Inclusion of PbO in a glass component of a solar cell paste has the
desirable effects of (a) lowering the firing temperature of paste
compositions, (b) facilitating interaction with the silicon
substrate and, upon firing, helping to form low resistance contacts
with silicon. For these and other reasons PbO is a significant
component in many conventional solar cell paste compositions.
However, in light of environmental concerns, the use of PbO (as
well as CdO), in paste compositions is now largely avoided whenever
possible. Hence a need exists in the photovoltaic industry for
lead-free and cadmium-free paste compositions, which afford
desirable properties using lead-free and cadmium-free glasses in
solar cell contact pastes.
SUMMARY OF THE INVENTION
The present invention provides lead-free and cadmium-free glass
compositions for use in solar cell contact paste materials that
provide low series resistance (R.sub.S) and high shunt resistance
(R.sub.sh) to give high performance solar cells, as measured by
efficiency (.eta.) and fill factor (FF). Generally, the present
invention includes a solar cell comprising a contact, made from a
mixture wherein, prior to firing, the mixture comprises a solids
portion and an organics portion. The solids portion comprises from
about 85 to about 99 wt % of a conductive metal component and from
about 1 to about 15 wt % of a lead-free glass component.
The compositions and methods of the present invention overcome the
drawbacks of the prior art by optimizing interaction, bonding, and
contact formation between contact components, typically silicon
with either Ag (front contact) or Al (back contact) or Ag (silver
rear contact), through the lead-free glass medium. A conductive
paste containing glass and silver, or glass and aluminum, is
printed on a silicon substrate, and fired to fuse the glass and
sinter the metal therein. For a silver rear contact, the metal
component may comprise silver, or a combination of silver and
aluminum powders and/or flakes. Upon firing, for a front contact,
Ag/Si conductive islands are formed providing conductive bridges
between bulk paste and silicon wafer. In a front contact, the
sequence and rates of reactions among glasses, metals and silicon,
occurring as a function of temperature are factors in forming the
low resistance contact between the silver paste and silicon wafer.
The interface structure consists of multiple phases: substrate
silicon, Ag/Si islands, Ag precipitates within the insulating glass
layer, and bulk silver. The glass forms a nearly continuous layer
between the silicon interface and the bulk silver. For a back
contact, upon firing, a p.sup.+ layer forms on the underlying
silicon by liquid-phase epitaxy. This occurs during the
resolidification of the aluminum-silicon (Al--Si) melt.
High-bismuth lead-free and cadmium-free glasses allow low firing
temperatures in making front contacts owing to their excellent flow
characteristics relatively at low temperatures. Relatively
high-silicon, low bismuth lead-free and cadmium-free glasses
provide suitable properties for back contacts, without excessive
interaction with backside Si. Similarly, high-bismuth lead-free and
cadmium-free glasses allow the formation of suitable lead-free
silver rear contacts on backside Si with optimal interaction with
both Si and back contact Al layer.
The foregoing and other features of the invention are hereinafter
more fully described and particularly pointed out in the claims,
the following description setting forth in detail certain
illustrative embodiments of the invention, these being indicative,
however, of but a few of the various ways in which the principles
of the present invention may be employed.
DETAILED DESCRIPTION OF THE INVENTION
Broadly, the invention provides a solar cell contact made from a
mixture wherein, prior to firing, the mixture comprises a solids
portion and an organics portion, wherein the solids portion
comprises from about 85 to about 99 wt %, preferably about 88 to
about 96 wt % of a conductive metal component, and from about 1 to
about 15 wt %, preferably about 2 to about 9 wt % and more
preferably about 3 to about 8 wt % of a glass component, wherein
the glass component is lead-free and cadmium-free. A solar panel
comprising any solar cell herein is also envisioned. When the solar
cell contact is a front contact, the metal component preferably
comprises silver, and the glass component comprises from about 5 to
about 85 mol % Bi.sub.2O.sub.3, and from about 1 to about 70 mol %
SiO.sub.2. The compositions used in making front contacts are also
useful in making a busbar (silver rear contact) for a solar cell
back contact. A silver (or silver-aluminum) rear contact in the
back makes contact with both Si and the Al back contact layer, even
though back contact Al also directly contacts Si. The silver rear
contact in the back contact helps to solder connecting tabs to the
solar cells that connect one cell to the next in a solar cell
module. In a back contact, the metal component preferably comprises
aluminum, and the glass component comprises from about 5 to about
55 mol % Bi.sub.2O.sub.3, from about 20 to about 70 mol %
SiO.sub.2, and from about 0.1 to about 35 mol % B.sub.2O.sub.3.
Broadly, silver- and glass-containing thick film pastes are used to
make front contacts for silicon-based solar cells to collect
current generated by exposure to light. While the paste is
generally applied by screen-printing, methods such as extrusion,
pad printing, and hot melt printing may also be used. Solar cells
with screen-printed front contacts are fired to relatively low
temperatures (550.degree. C. to 850.degree. C. wafer temperature;
furnace set temperatures of 650.degree. C. to 1000.degree. C.) to
form a low resistance contact between the N-side of a phosphorus
doped silicon wafer and a silver based paste. Methods for making
solar cells are also envisioned herein.
Aluminum- and glass-containing back contacts are used to form low
resistance ohmic contacts on the back side of the solar cell due to
large area melting and re solidification of Al doped (p.sup.+)
epitaxially grown Si layer which increases the solar cell
performance due to improved back surface field. For optimum
performance a thick p.sup.+ re-grown region is believed to be
ideal. It is also believed that the rejection of metallic
impurities from the epitaxially growing p.sup.+ layer leads to high
carrier lifetimes. These two factors are believed to increase the
open circuit voltage, and more importantly, the open circuit
voltage falls only slightly as the bulk resistivity increases.
Therefore solar cell performance improves due to the formation of
substantial epitaxially re grown p.sup.+ layer in the Al back
contact. Therefore the interaction of lead-free and cadmium-free
glass in the back contact paste, with Si should be minimal, and its
interaction with Al should be enough to form a continuous Al layer
without beading.
Paste Glasses. The glass component of the pastes comprises, prior
to firing, one or more glass compositions. Each glass composition
comprises oxide frits including, at a minimum, Bi.sub.2O.sub.3 and
SiO.sub.2. In particular, in various embodiments of the present
invention, glass compositions for a front contact may be found in
Table 1. Glass compositions for back contacts may be found in Table
2. More than one glass composition can be used, and compositions
comprising amounts from different columns in the same table are
also envisioned. Regardless of the number of glass compositions
used, the total content of Bi.sub.2O.sub.3 and SiO.sub.2 in the
glass component preferably falls within the range of about 5 to
about 85 mol % Bi.sub.2O.sub.3 and from about 1 to about 70 mol %
SiO.sub.2. If a second glass composition is used, the proportions
of the glass compositions can be varied to control the extent of
paste interaction with silicon, and hence the resultant solar cell
properties. For example, within the glass component, the first and
second glass compositions may be present in a weight ratio of about
1:20 to about 20:1, and preferably about 1:3 to about 3:1. The
glass component preferably contains no lead or oxides of lead, and
no cadmium or oxides of cadmium.
TABLE-US-00001 TABLE 1 Oxide frit ingredients for front contact
glasses in mole percent. Glass Composition Ingredient I II III
Bi2O3 5 85 15 80 50 80 SiO.sub.2 1 70 2 45 15 35 ZnO 0 55 0.1 25 1
15 V.sub.2O.sub.5 0 30 0.1 25 1 15
TABLE-US-00002 TABLE 2 Oxide frit ingredients for back contact
glasses in mole percent. Glass Composition Ingredient IV V VI
Bi.sub.2O.sub.3 5 65 5 55 10 40 SiO.sub.2 15 70 20 70 30 65
B.sub.2O.sub.3 0 35 0.1 35 3 20 Alkali oxides 0 35 0.1 25 5 25
In addition to the oxides of Table 1 and Table 2, additional oxides
may be included in the glass component, for example about 1 to
about 20 mol % of a trivalent oxide of one or more of Al, B, La, Y,
Ga, In, Ce, and Cr; about 0.1 to about 15 mol % of a tetravalent
oxide of one or more of Ti, Zr and Hf; about 0.1 to about 20 mol %
of a pentavalent oxide of one or more of P, Ta, Nb, and Sb.
Ag.sub.2O may be included in the silver paste glass as a source of
silver, from about 0.1 to about 12 mol %.
Metal Component. In a solar cell contact, the metal must be
conductive. In a front contact, the metal component comprises
silver. The source of the silver can be one or more fine powders of
silver metal, or alloys of silver. A portion of the silver can be
added as silver oxide (Ag.sub.2O) or as silver salts such as silver
chloride (AgCl), silver nitrate (AgNO.sub.3) or silver acetate
(AgOOCCH.sub.3). The silver particles used in the paste may be
spherical, flaked, or provided in a colloidal suspension, and
combinations of the foregoing may be used. For example the solids
portion of the paste may comprise about 80 to about 99 wt %
spherical silver particles or alternatively about 75 to about 90 wt
% silver particles and about 1 to about 10 wt % silver flakes.
Alternatively the solids portion may comprise about 75 to about 90
wt % silver flakes and about 1 to about 10 wt % of colloidal
silver, or about 60 to about 95 wt % of silver powder or silver
flakes and about 0.1 to about 20 wt % of colloidal silver. Suitable
commercial examples of silver particles are spherical silver powder
Ag3000-1, silver flakes SF-29, and colloidal silver suspension
RDAGCOLB, all commercially available from Ferro Corporation,
Cleveland, Ohio.
In a back contact, the metal component comprises aluminum or alloys
of aluminum. The aluminum metal component may come in any suitable
form, including those noted hereinabove for silver in the front
contact.
For a silver rear contact, the metal component may comprise silver
or a combination of both silver and aluminum pastes as disclosed
hereinabove.
Other Additives. Up to about 30wt % of other (i.e., inorganic)
additives, preferably up to about 25 wt % and more preferably up to
about 20 wt %, may be included as needed. Phosphorus can be added
to the paste in a variety of ways to reduce the resistance of the
front contacts. For example, certain glasses can be modified with
P.sub.2O.sub.5 in the form of a powdered or fritted oxide, or
phosphorus can be added to the paste by way of phosphate esters or
other organo-phosphorus compounds. More simply, phosphorus can be
added as a coating to silver particles prior to making a paste. In
such case, prior to pasting, the silver particles are mixed with
liquid phosphorus and a solvent. For example, a blend of from about
85 to about 95 wt % silver particles, from about 5 to about 15 wt %
solvent and from about 0.5 to about 10 wt % liquid phosphorus is
mixed and the solvent evaporated. Phosphorus coated silver
particles help ensure intimate mixing of phosphorus and silver in
the inventive silver pastes.
Other additives such as fine silicon or carbon powder, or both, can
be added to control the reactivity of the metal component with
silicon. For example these fine silicon or carbon powder can be
added to the front contact silver paste to control the silver
reduction and precipitation reaction. The silver precipitation at
the Ag/Si interface or in the bulk glass, for the silver pastes in
both front contacts and silver rear contacts, can also be
controlled by adjusting the firing atmosphere (e.g., firing in
flowing N.sub.2 or N.sub.2/H.sub.2/H.sub.2O mixtures). Fine
particles of low melting metal additives (i.e., elemental metallic
additives as distinct from metal oxides) such as Pb, Bi, In, Ga,
Sn, and Zn and alloys of each with at least one other metal can be
added to provide a contact at a lower temperature, or to widen the
firing window. Zinc is the preferred metal additive, and a
zinc-silver alloy is most preferred for the front contact.
A mixture of (a) glasses or a mixture of (b) crystalline additives
and glasses or a mixture of (c) one or more crystalline additives
can be used to formulate a glass component in the desired
compositional range. The goal is to reduce the contact resistance
and improve the solar cell electrical performance. For example,
second-phase crystalline materials such as Bi.sub.2O.sub.3,
Sb.sub.2O.sub.3, ln.sub.2O.sub.3, Ga.sub.2O.sub.3, SnO, ZnO,
SiO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, B.sub.2O.sub.3,
V.sub.2O.sub.5, Ta.sub.2O.sub.5, various alumino-silicates, bismuth
borates, bismuth silicates such as 6Bi.sub.2O.sub.3.SiO.sub.2,
Bi.sub.2O.sub.3.SiO.sub.2, 2Bi.sub.2O.sub.3.3SiO.sub.2,
12Bi.sub.2O.sub.3.SiO.sub.2, 2Bi.sub.2O.sub.3.SiO.sub.2,
3Bi.sub.2O.sub.3.5SiO.sub.2 and Bi.sub.2O.sub.3.4SiO.sub.2, bismuth
titanantes such as Bi.sub.2O.sub.3.2TiO.sub.2, 2Bi.sub.2O.sub.3.
3TiO.sub.2, 2Bi.sub.2O.sub.3.4TiO.sub.2, and
6Bi.sub.2O.sub.3.TiO.sub.2, various vanadates such as
MgO.V.sub.2O.sub.5, CaO.V.sub.2O.sub.5, BaO.V.sub.2O.sub.5,
Zn.V.sub.2O.sub.5, Na.sub.2O.17V.sub.2O.sub.5,
K.sub.2O.4V.sub.2O.sub.5, 2Li.sub.2O.5V.sub.2O.sub.5, and bismuth
vanatades such as 6Bi.sub.2O.sub.3.V.sub.2O.sub.5, BiVO.sub.4,
2Bi.sub.2O.sub.3.3V.sub.2O.sub.5, and BiV.sub.3O.sub.9, bismuth
vanadium titanates such as
6.5Bi.sub.2O.sub.3.2.5V.sub.2O.sub.5.Tio.sub.2, zinc titanates such
as 2ZnO.3TiO.sub.2, zinc silicates such as ZnO.SiO.sub.2, zirconium
silicates such as ZrO.sub.2.SiO.sub.2, and reaction products
thereof and combinations thereof may be added to the glass
component to adjust contact properties. However, the total amounts
of the above oxides must fall within the ranges specified for
various embodiments disclosed elsewhere herein.
Organic Vehicle. The pastes herein include a vehicle or carrier
which is typically a solution of a resin dissolved in a solvent
and, frequently, a solvent solution containing both resin and a
thixotropic agent. The organics portion of the pastes comprises (a)
at least about 80 wt % organic solvent; (b) up to about 15 wt % of
a thermoplastic resin; (c) up to about 4 wt % of a thixotropic
agent; and (d) up to about 2 wt % of a wetting agent. The use of
more than one solvent, resin, thixotrope, and/or wetting agent is
also envisioned. Although a variety of weight ratios of the solids
portion to the organics portion are envisioned, one embodiment
includes a weight ratio of the solids portion to the organics
portion from about 20:1 to about 1:20, preferably about 15:1 to
about 1:15, and more preferably about 10:1 to about 1:10.
Ethyl cellulose is a commonly used resin. However, resins such as
ethyl hydroxyethyl cellulose, wood rosin, mixtures of ethyl
cellulose and phenolic resins, polymethacrylates of lower alcohols
and the monobutyl ether of ethylene glycol monoacetate can also be
used. Solvents having boiling points (1 atm) from about 130.degree.
C. to about 350.degree. C. are suitable. Widely used solvents
include terpenes such as alpha- or beta-terpineol or higher boiling
alcohols such as Dowanol.RTM. (diethylene glycol monoethyl ether),
or mixtures thereof with other solvents such as butyl Carbitol.RTM.
(diethylene glycol monobutyl ether); dibutyl Carbitol.RTM.
(diethylene glycol dibutyl ether), butyl Carbitol.RTM. acetate
(diethylene glycol monobutyl ether acetate), hexylene glycol,
Texanol.RTM. (2,2,4-trimethyl-1,3-pentanediol monoisobutyrate), as
well as other alcohol esters, kerosene, and dibutyl phthalate. The
vehicle can contain organometallic compounds, for example those
based on nickel, phosphorus or silver, to modify the contact.
N-DIFFUSOL.RTM. is a stabilized liquid preparation containing an
n-type diffusant with a diffusion coefficient similar to that of
elemental phosphorus. Various combinations of these and other
solvents can be formulated to obtain the desired viscosity and
volatility requirements for each application. Other dispersants,
surfactants and rheology modifiers, which are commonly used in
thick film paste formulations, may be included. Commercial examples
of such products include those sold under any of the following
trademarks: Texanol.RTM. (Eastman Chemical Company, Kingsport,
Tenn.); Dowanol.RTM. and Carbitol.RTM. (Dow Chemical Co., Midland,
Mich.); Triton.RTM. (Union Carbide Division of Dow Chemical Co.,
Midland, Mich.), Thixatrol.RTM. (Elementis Company, Hightstown
N.J.), and Diffusol.RTM. (Transene Co. Inc., Danvers, Mass.).
Among commonly used organic thixotropic agents is hydrogenated
castor oil and derivatives thereof. A thixotrope is not always
necessary because the solvent coupled with the shear thinning
inherent in any suspension may alone be suitable in this regard.
Furthermore, wetting agents may be employed such as fatty acid
esters, e.g., N-tallow-1,3-diaminopropane dioleate; N-tallow
trimethylene diamine diacetate; N-coco trimethylene diamine, beta
diamines; N-oleyl trimethylene diamine; N-tallow trimethylene
diamine; N-tallow trimethylene diamine dioleate, and combinations
thereof.
It should be kept in mind that the foregoing compositional ranges
are preferred and it is not the intention to be limited to these
ranges where one of ordinary skill in the art would recognize that
these ranges may vary depending upon specific applications,
specific components and conditions for processing and forming the
end products.
Paste Preparation. The paste according to the present invention may
be conveniently prepared on a three-roll mill. The amount and type
of carrier utilized are determined mainly by the final desired
formulation viscosity, fineness of grind of the paste, and the
desired wet print thickness. In preparing compositions according to
the present invention, the particulate inorganic solids are mixed
with the vehicle and dispersed with suitable equipment, such as a
three-roll mill, to form a suspension, resulting in a composition
for which the viscosity will be in the range of about 100 to about
500 kcps, preferably about 300 to about 400 kcps, at a shear rate
of 9.6 sec.sup.-1 as determined on a Brookfield viscometer HBT,
spindle 14, measured at 25.degree. C.
Printing and Firing of the Pastes. The aforementioned paste
compositions may be used in a process to make a solar cell contact
or other solar cell components. The inventive method of making
solar cell front contact comprises (1) applying a silver-containing
paste to the silicon substrate, (2) drying the paste, and (3)
firing the paste to sinter the metal and make contact to silicon.
The printed pattern of the paste is fired at a suitable
temperature, such as about 650-950.degree. C. furnace set
temperature, or about 550-850.degree. C. wafer temperature.
Preferably, the furnace set temperature is about 750-930.degree.
C., and the paste is fired in air. During the firing the
antireflective SiN.sub.X layer is believed to be oxidized and
corroded by the glass and Ag/Si islands are formed on reaction with
the Si substrate, which are epitaxially bonded to silicon. Firing
conditions are chosen to produce a sufficient density of Ag/Si
islands on the silicon wafer at the silicon/paste interface,
leading to a low resistivity, high efficiency, high-fill factor
front contact and solar cell.
The lead-free silver pastes herein can also be used to form a
backside Ag silver rear contact. A method of making a backside Ag
silver rear contact comprises: (1) applying a silver paste to the
P-side of a silicon wafer in bus-bar configuration, (2) drying the
paste, (3) printing and drying a Al-back contact paste, (4)
applying and drying the above mentioned silver front contact paste,
and (5) co-firing all three pastes, at a suitable temperature, such
as about 650-950.degree. C. furnace set temperature; or about
550-850.degree. C. wafer temperature.
The inventive method of making solar cell back contact comprises:
(1) applying an Al-containing paste to the P-side of a silicon
wafer on which back silver rear contact paste is already applied
and dried, (2) drying the paste, and (3) applying the front contact
silver paste, and (4) co-firing the front contact, silver rear
contact, and Al-back contact. The solar cell printed with silver
rear contact Ag-paste, Al-back contact paste, and Ag-front contact
paste is fired at a suitable temperature, such as about
650-950.degree. C. furnace set temperature; or about
550-850.degree. C. wafer temperature. During firing Al as the wafer
temperature rises above Al--Si eutectic temperature of 577.degree.
C., the back contact Al dissolves Si from the substrate and liquid
Al--Si layer is formed. This Al--Si liquid continues to dissolve
substrate Si into it during further heating to peak temperature.
During the cool down period, Si precipitates back from Al--Si melt.
This precipitating Si grows as an epitaxial layer on the underlying
Si substrate and forms a purer p.sup.+ layer. When the cooling melt
reaches Al--Si eutectic temperature the remaining liquid freezes as
Al--Si eutectic layer. A purer P+ layer is believed to provide a
back surface field (BSF), which in turn increases the solar cell
performance. So the glass in Al-back contact should optimally
interact with both Al and Si without unduly affecting the formation
of an efficient BSF layer.
A typical ARC is made of a silicon compound such as silicon
nitride, generically SiN.sub.X, such as Si.sub.3N.sub.4, and it is
generally on the front contact side of silicon substrate. This ARC
layer acts as an insulator, which tends to increase the contact
resistance. Corrosion of this ARC layer by the glass component is
hence a necessary step in front contact formation. Reducing the
resistance between the silicon wafer and the paste improves solar
cell efficiency and is facilitated by the formation of epitaxial
silver/silicon conductive islands at the front contact Ag/Si
interface. That is, the silver islands on silicon assume the same
crystalline structure as is found in the silicon substrate. Until
now, the processing conditions to achieve a low resistance
epitaxial silver/silicon interface have involved the use of Ag
pastes that contain leaded glasses. The lead free Ag-pastes and
processes herein now make it possible to produce an epitaxial
silver/silicon interface leading to a contact having low resistance
under broad processing conditions--a firing temperature as low as
about 650.degree. C., and as high as about 850.degree. C. (wafer
temperature)--to produce lead free front contacts. The lead-free
pastes herein can be fired in air; i.e., where no special
atmospheric conditions are required.
The formation of a low resistance lead-free front contact on a
silicon solar cell is technically challenging. Both the
interactions among paste constituents (silver metal, glass,
additives, organics), and the interactions between paste
constituents and silicon substrate are complex, yet must be
controlled. The rapid furnace processing makes all the reactions
highly dependent on kinetics. Further, the reactions of interest
must take place within a very narrow region (<0.5 micron) of
silicon in order preserve the P-N junction. Similarly the formation
of lead-free back contacts on a silicon solar cell is technically
challenging.
Method of Front Contact Production. A solar cell front contact
according to the present invention can be produced by applying any
Ag paste produced by mixing silver powders with lead free and
cadmium-free glasses disclosed in Table 1 to the N-side of the
silicon substrate pre coated with back Ag silver rear contact paste
and Al back contact paste, for example by screen printing, to a
desired wet thickness, e.g., from about 40 to 80 microns.
Method of Silver Rear Contact Production. A solar cell silver rear
contact according to the present invention can be produced by
applying any Ag paste produced by mixing silver or silver alloy
powders with lead free glasses disclosed in Table 1 to the P-side
of the silicon substrate, for example by screen printing, to a
desired wet thickness, e.g., from about 40 to 80 microns.
Method of Back Contact Production. A solar cell back contact
according to the present invention can be produced by applying any
Al paste produced by mixing aluminum powders with lead free glasses
disclosed in Table 2 to the P-side of the silicon substrate pre
coated with silver rear contact paste, for example by screen
printing, to a desired wet thickness, e.g., from about 30 to 50
microns.
Common to the production of front contacts, back contacts and
silver rear contacts is the following. Automatic screen printing
techniques can be employed using a 200-325 mesh screen. The printed
pattern is then dried at 200.degree. C. or less, preferably at
about 120.degree. C. for about 5-15 minutes before firing. The dry
printed pattern can be co fired with silver rear contact and Al
back contact pastes for as little as 1 second up to about 5 minutes
at peak temperature, in a belt conveyor furnace in air.
Nitrogen (N.sub.2) or another inert atmosphere may be used if
desired, but it is not necessary. The firing is generally according
to a temperature profile that will allow burnout of the organic
matter at about 300.degree. C. to about 550.degree. C., a period of
peak furnace set temperature of about 650.degree. C. to about
1000.degree. C., lasting as little as about 1 second, although
longer firing times as high as 1, 3, or 5 minutes are possible when
firing at lower temperatures. For example a three-zone firing
profile may be used, with a belt speed of about 1 to about 4 meters
(40-160 inches) per minute. Naturally, firing arrangements having
more than 3 zones are envisioned by the present invention,
including 4, 5, 6, or 7, zones or more, each with zone lengths of
about 5 to about 20 inches and firing temperatures of 650 to
1000.degree. C.
EXAMPLES
Polycrystalline silicon wafers, 12.5 cm.times.12.5 cm, thickness
250-300 .mu.m, were coated with a silicon nitride antireflective
coating on the N-side of Si. The sheet resistivity of these wafers
was about 1 .OMEGA.-cm. Exemplary lead-free and cadmium-free
glasses of this invention are listed in Table 3.
TABLE-US-00003 TABLE 3 Exemplary Glass Compositions Glass G I J L M
Mole % Bi.sub.2O.sub.3 60 60 75 35.8 21.57 SiO.sub.2 35 30 20 35.5
43.9 ZnO 5 9.7 B.sub.2O.sub.3 7.2 10.0 Al.sub.2O.sub.3 10
V.sub.2O.sub.5 5 Li.sub.2O 10.5 Na.sub.2O 2.5 K.sub.2O 21.5
Nb.sub.2O.sub.5 1.86
Exemplary Ag- or Al-paste formulations in Table 4 were made with
commonly used 2.about.5 .mu.m silver powders or flakes and
4.about.10 .mu.m aluminum powders, and the organic vehicles V131,
V132, V148, V205, and V450 commercially available from Ferro
Corporation, Cleveland, Ohio. N-Diffusol is commercially available
from Transene Co. Inc., Danvers, Mass. Anti-Terra 204 is a wetting
agent commercially available from BYK-Chemie GmbH, Wesel, Germany.
Cabosil.RTM. is fumed silica, commercially available from Cabot
Corporation, Billerica Mass. All amounts in Table 4 are in weight
percent of the paste, including the solids portion and the organics
portion.
TABLE-US-00004 TABLE 4 Exemplary Pb-free Paste Formulations Paste 1
2 3 4 Type front front back silver rear contact Glass component
Ingredients in wt % I J L M Glass component in paste 4.7 4.5 1.6 5
Silver 80.9 78.0 69.9 Aluminum 78.2 Cabosil 0.4 Vehicle V131 1.1
3.5 10.4 Vehicle V132 8.8 13.5 14.7 Vehicle V148 4.1 Vehicle V205
7.25 Vehicle V450 3.75 Texanol 7.8 Anti-Terra 204 1.0 N-diffusol
0.4 0.5
The exemplary lead-free pastes in Table 4 were printed either as
front contact or back silver rear contact or back contact on a
silicon solar cell and their solar cell properties are compared to
the prior art lead containing pastes as shown in Table 5. The other
two pastes were accordingly commercially available front contact
(CN33-462) or silver rear contact (3368, 33-451, or 33-466) or Al
back contact (FX53-038, or CN53-100 or CN53-101) pastes from Ferro
corporation, Cleveland, Ohio. The front contact pattern was printed
using a 280 mesh screen with 100 .mu.m openings for finger lines
and with about 2.8 mm spacing between the lines. The silver rear
contact and back contact pastes were printed using 200 mesh screen.
The printed wafers were co-fired using a 3-zone infrared (IR) belt
furnace with a belt speed of about 3 meters (120'') per minute,
with temperature settings of 780.degree. C., 810.degree. C., and
930 to 970.degree. C. for the three zones. The zones were 7'',
16'', and 7'' long, respectively. For the front contact Ag lines
the fired finger width for most samples was about 120 to 170 .mu.m
and the fired thickness was about 10 to 15 .mu.m.
These lead free pastes and their comparative prior art lead pastes
were fired side by side according to the aforementioned firing
profile. Electrical performance of these solar cells was measured
with a solar tester, Model 91193-1000, Oriel Instrument Co.,
Stratford, Conn., under AM 1.5 sun conditions, in accordance with
ASTM G-173-03. The electrical properties of the resultant solar
cells are set forth in Table 5.
TABLE-US-00005 TABLE 5 Properties of Solar cells made with Pb-free
pastes of Table 4 compared to the corresponding prior art lead
containing pastes. Paste Prior Prior Prior art art art Prior CN33-
CN33- FX53- art 1 462 2 462 3 038 4 3398 PasteType Lead free leaded
Leaded Leaded Leaded Glass I J L M Glass Type Pb-free Pb Pb-free Pb
Pb-free Pb Pb-free Pb Isc, A 5.653 5.718 5.087 5.177 4.966 5.079
4.942 4.920 Voc, mV 601 609 610 609 600 606 606 603 Efficiency, %
13.10 15.49 14.91 15.18 13.96 13.39 13.6 13.4 Fill Factor, % 59.8
69.0 75.0 75.1 73.1 67.7 70.6 70.5 Rs, m.OMEGA. 21 10.0 8.8 8.0
11.0 14.0 14.0 13.0 Rsh, .OMEGA. 3.47 4.12 12.8 17.3 9.45 5.88 8.0
6.7
The prior art pastes 3398, CN33-462, FX53-038 are commercially
available from Ferro Corporation, Cleveland, Ohio. Isc means short
circuit current, measured at zero output voltage; Voc means open
circuit voltage measured at zero output current; R.sub.S and
R.sub.sh were previously defined. The terms Efficiency and Fill
Factor are known in the art.
Table 5 clearly shows the invented lead free pastes give solar cell
properties comparable to appropriate prior art lead containing
pastes. Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
illustrative example shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general invention concept as defined by the
appended claims and their equivalents.
* * * * *